Spectrophotometric-type instruments are known and used in a variety of applications. An instrument of this type is, for example, disclosed in the Anderson et al. U.S. Pat. No. 5,879,294. There remains, however, a continuing need for instruments capable of providing measurement to a higher degree of accuracy with relatively low levels of output signal drift.
An improvement to the spectrophotometric type instrument was the invention of U.S. Pat. No. 6,377,840 (Gritsenko et al.) wherein a reference light measurement was used improve the output signal. Prior art FIG. 1 shows a block diagram of wherein an instrument 10 includes an optical probe 12 which is releasably connected to an electronics package 14 via optical fibers 16. The electronics package 14 includes a connector 18, a detector 20, a processor/controller 22, and a display 24. In operation, the probe 12 is positioned on the tissue to be measured or analyzed. The probe 12 is interfaced to the instrument electronics through the optical fibers 16 and a probe connector 26. The probe connector 26 includes light emitting diodes (LEDs) or other light sources 30, 32, 34, 36, and 38 for generating light at a number of different wavelengths (e.g., 800, 760, 720, 680, and 530 nm, respectively). The light used to measure the characteristics of the tissue is coupled to the probe by send optical fibers 40, 42, 44, and 46. After being transmitted from the tissue-engaging surface of the probe 12 into the tissue being measured, the light will travel through the tissue before being collected at the end of the receive optical fiber 48. This collected light (measurement light signal) is then transmitted to the instrument 14 through the probe connector 26 and electronics package connector 18. A reference light signal corresponding to each of the measurement light signals (i.e., the reference light signals are not transmitted through the tissue) is also transmitted to the electronics package connector 18.
The collected measurement light signals and reference light signals received by the electronics package 14 are transmitted to the detector 20 which produces electrical signals representative of these light signals at each wavelength of interest. The processor/controller 22 then processes these signals to generate data representative of the measured tissue parameter (e.g., saturated oxygen level (StO2)). The measurement reading can be visually displayed on the display 24. Algorithms used to compute the tissue parameter data are generally known and described in the Anderson et al. U.S. Pat. No. 5,879,294.
Calibration procedures are typically performed to enhance the accuracy of the measurements subsequently made by the instrument 14. Methods and devices for calibrating spectrophotometric-type instruments are generally known and disclosed in the Anderson et al. patent. The calibration can, for example, be performed by placing the probe 12 on a calibration device 50 such as that shown in prior art FIG. 1. The calibration device 50 includes a housing, which is filled with light scattering material. The light scattering material is generally spectrally flat (i.e., reflects all light to the same degree) to provide a reference spectrum. White polyethylene foam such as Plastazote LD45 available from Zotefoams plc. can be used for this purpose.
One configuration of a spectrophotometric instrument of the type described above includes, for each wavelength of interest, a photomultiplier tube (PMT) for detecting the measurement light signal, and a photodiode for detecting the calibration recognition signal (or ambient light). Thermal electric coolers can be included in the electronics package to help maintain temperature control of the optical bench to which the PMTs and photodiodes are mounted, and thereby reduce output signal drift.
The present invention is an optical bench configuration, measurement and reference signal acquisition system and measurement and reference signal processing algorithm which provide relatively low levels of output signal drift. The probe connector 26 used in connection with this invention is illustrated in prior art FIG. 2, which shows an embodiment having a reference signal generated within the connector. As shown, the probe connector 26 includes 4 LEDs 30, 32, 34, and 36 for generating the measurement light signals at 800, 760, 720 and 680 nm. Light signals from each of these LEDs are coupled to the probe 12 by a separate measurement signal send fiber 40, 42, 44, 46. After being transmitted through the tissue being analyzed and collected at the probe, the measurement light signal is coupled back to the probe connector by a measurement signal receive fiber 16C. The end of the measurement signal receive fiber 16C terminates in the probe connector 26 at a sample ferrule 52 which is adapted to mate with a socket in the connector 18 of the electronics package 14.
The probe connector 26 also provides a reference light signal. The reference light signal includes a portion of the light from each of the LEDs, and has not been transmitted from the probe before being collected. In the embodiment shown in prior art FIG. 2, the reference light signal is collected by reference light signal send optical fibers 54, 56, 58, and 60 which extend respectively from each measurement light signal source LED 30, 32, 34, 36. The reference light received from each LED is mixed using a mixer 62 and transmitted through the reference signal fiber 16B. The end of the reference signal fiber 16B terminates in the probe connector 26. Since it is significantly attenuated when it is transmitted through the tissue, the intensity of the measurement light signal at the connector is much less than the intensity of the non-attenuated reference light signal (e.g., about 1 million times less). In order to match the reference and measurement signal magnitudes to enable detection with a similar photo multiplier tube gain, the reference signal is attenuated at the mixer 62. The reference signal attenuation is obtained by reflectance mode positioning the reference signal send fibers 54, 56, 58, 60 equidistant from the centrally located reference signal receive fiber 16B. The concentration of scattering material (such as titanium dioxide from Aldrich, Milwaukee, Wis.) within an optically clear epoxy substrate (such as EpoTech 301 from Epoxy Technology, Billerica, Mass.) can be adjusted to provide the appropriate level of attenuation within the mixer 62.
Light transmitted from the probe tip 12 from send fiber 16A is collected through the probe tip by receive fiber 16C which may also be connected to the monitor by probe connector 26.
In the monitor, a tissue value represented by the reflected light intensity and wavelength distribution can be determined. The received light signal is directed to the detector or optical bench 20 for separation into selected component wavelengths that are then passed to a processor 22 for processing. Similarly, the reference light signal is directed to the optical bench for separation into its component wavelengths. The reference light signal is used to correct the value of the received light signal.
The same optical bench is used for measuring both sample and reference signals to compensate for the drift. Accordingly, a shutter system 80 was used to alternately permit or prohibit one of light signals from reaching the optical bench when the other light signal is being analyzed. The prior art shutter system 80 included motor 87, and shutter 84. The shutter 84 was shaped so that when one light transmission path was blocked, the other path could transmit light through aperture plate 86. Motor 87 positioned the shutter to prevent passage of light by one of the two light signals.